1. Field of the Invention
The present invention relates to a voltage controlled oscillator which utilizes resonance of a parallel LC tank circuit. In particular, the invention relates to a voltage controlled oscillator which has a capacitance switch and is capable of changing its oscillation frequency stepwise.
2. Description of the Related Art
Conventionally, voltage controlled oscillators (VCOs) such as a ring oscillator have been used as local oscillators (LOs) in phase-locked loop (PLL) circuits which are used for frequency multiplication and phase synchronization.
For such local oscillators, voltage controlled oscillators utilizing resonance of a parallel LC tank circuit (LC-VCOs) have been used recently. An LC-VCO has an inductor and a variable capacitor which are connected in parallel to form a parallel LC tank circuit. This parallel LC tank circuit causes resonance to oscillate, or output, an alternating-current signal having the resonant frequency. The resonant frequency refers to a frequency at which the parallel LC tank circuit exhibits infinite impedance. The resonance refers to the phenomenon that a current flows through the inductor and the variable capacitor of the parallel LC circuit alternately. The variable capacitor is composed of varactor devices or the like, and varies in capacitance depending on a control voltage applied thereto. The resonant frequency f is given by the following equation 1:                     f        =                  1                      2            ⁢            π            ⁢                          LC                                                          (                  Equation          ⁢                                           ⁢          1                )            where L is the inductance of the inductor, and C is the capacitance of the variable capacitor. The foregoing equation 1 shows that the capacitance C of the variable capacitor can be increased to lower the resonant frequency f.
As compared to the conventional VCOs using a ring oscillator or the like, the LC-VCO has the following advantages. Firstly, the LC-VCO produces smaller noise as compared to the conventional VCOs using a ring oscillator or the like. The reason for this is that the basic principle of the LC-VCO, i.e., the resonance of the parallel LC tank circuit requires a smaller number of transistors which can cause noise. The LC-VCO is thus suitable for high-speed optical communications, cellular phones, wireless LANs, etc. Secondly, since its basic principle lies in LC-circuit resonance, the LC-VCO can provide high oscillation frequencies more easily than VCOs that are made of transistors and utilize logic-gate delays. Thirdly, the LC-VCO has a narrower range of variation in oscillation frequency with respect to the control voltage. This means a lower tuning sensitivity and less variations of the oscillation frequency resulting from fluctuations in the control voltage, with the result of lower noise.
In the meantime, one of the drawbacks of the LC-VCO is also the low tuning sensitivity mentioned above. While it is advantageous in terms of noise as described above, the low tuning sensitivity narrows the range of variation in oscillation frequency and thus makes it difficult to design LC-VCOs of desired oscillation frequencies.
To overcome this drawback, LC-VCOs having a capacitance switch have been proposed (for example, see A. Kral et al., “RF-CMOS Oscillators with Switched Tuning”, IEEE Custom Integrated Circuits Conf., pp. 555-558, 1998). FIG. 1 is an equivalent circuit diagram showing a conventional LC-VCO having a capacitance switch. FIG. 2 is a graph for showing the range of variation in the oscillation frequency of this conventional LC-VCO, in which the abscissa indicates the control voltage applied to the variable capacitor and the ordinate indicates the oscillation frequency of the LC-VCO.
As shown in FIG. 1, this conventional LC-VCO 101 is connected to a power supply potential line VCC and a ground potential line GND. The LC-VCO. 101 has a negative resistance unit 2, an LC circuit unit 104, and a negative resistance unit 3 which are arranged in this order from the power supply potential line VCC to the ground potential line GND.
The negative resistance unit 2 has P-channel transistors 5 and 6. Either one of the source and drain of the P-channel transistor 5 is connected to the power supply potential line VCC. The other of the source and drain is connected to an output terminal 7 of the LC circuit unit 104. The gate is connected to an output terminal 8 of the same. Either one of the source and drain of the P-channel transistor 6 is connected to the power supply potential line VCC. The other of the source and drain is connected to the output terminal 8 of the LC circuit unit 104. The gate is connected to the output terminal 7.
The LC circuit 104 has an inductor 9 between the output terminals 7 and 8. A series of variable capacitors 10 and 11 is also connected between the output terminals 7 and 8, in parallel with the inductor 9. The variable capacitors 10 and 11 are capacitors which vary in capacitance according to a control voltage input thereto, or namely, varactor devices. The LC circuit unit 104 also has a capacitance switch unit 116. The capacitance switch unit 116 has capacitors 112 and 113, and switches 14 and 15. The output terminal 7 is connected to one electrode 112b of the capacitor 112. The other electrode 112a of this capacitor 112 is connected to one of the terminals of the switch 14. The other terminal of this switch 14 is connected to a ground electrode. That is, the switch 14 is intended to switch the electrode 112a between being connected to the ground electrode and floating. Similarly, the output terminal 8 is connected to the ground electrode via the capacitor 113 and the switch 15. That is, the switch 15 is intended to switch an electrode 113a of the capacitor 113 between being connected to the ground electrode and floating. The switches 14 and 15 are made of an N-channel transistor each.
The negative resistance unit 3 has N-channel transistors 17 and 18. Either one of the source and drain of the N-channel transistor 17 is connected to the output terminal 7 of the LC circuit unit 104. The other of the source and drain is connected to the ground potential line GND. The gate is connected to the output terminal 8. Either one of the source and drain of the N-channel transistor 18 is connected to the output terminal 8. The other of the source and drain is connected to the ground potential line GND. The gate is connected to the output terminal 7.
Next, description will be given of the operation of this conventional LC-VCO 101. When the LC circuit unit 104 is subjected to any electrical stimulation, for example, by the LC-VCO 101 being connected to the power supply potential line VCC and the ground potential line GND, alternative-current signals having the resonant frequency of the LC circuit unit 104 are output from the output terminals 7 and 8. In this case, the signals output from the output terminals 7 and 8 are complementary signals.
The LC circuit 104 by itself cannot sustain the oscillations due to a current loss resulting from its parasitic resistance. Then, a positive power supply potential is applied to the power supply potential line VCC and a ground potential is applied to the ground potential line GND to supply the LC-VCO 101 with an electric current. This combines with the provision of the negative resistance units 2 and 3 to allow permanent oscillation of resonant waves from the LC circuit unit 104.
More specifically, for example, when the output terminal 7 is low level and the output terminal 8 is high level, the P-channel transistor 5 turns off and the N-channel transistor 17 turns on. As a result, the ground potential is applied to the output terminal 7. Since the P-channel transistor 6 turns on and the N-channel transistor 18 turns off, the power supply potential is applied to the output terminal 8. Similarly, when the output terminal 7 is high level and the output terminal 8 is low level, the power supply potential is applied to the output terminal 7 and the ground potential is applied to the output terminal 8. This sustains the oscillations from the output terminals 7 and 8 without attenuation.
Then, the control voltage applied to the variable capacitors 10 and 11 is changed to vary the capacitances of the variable capacitors 10 and 11 continuously. Consequently, as shown in FIG. 2, the resonant frequency of the LC circuit unit 104 varies with the control voltage, allowing a change in the frequency of the alternating-currents output from the LC-VCO 101.
The switches 14 and 15 can also be switched to change the total capacitance of the capacitance switch unit 116. When the switch 14 is turned off, the electrode 112a of the capacitor 112 at the side of the switch 14 enters a high impedance state, and becomes almost the same as the electrode 112b of the capacitor 112 at the side of the output terminal 7 in potential. This prevents the capacitor 112 from functioning as a capacitance. Similarly, when the switch 15 is turned off, the electrode 113a enters a high impedance state to prevent the capacitor 113 from functioning as a capacitance. Consequently, turning off the switches 14 and 15 decreases the total capacitance of the LC circuit unit 104 and, from the foregoing equation 1, increases the oscillation frequency.
On the other hand, when the switch 14 is turned on, the electrode 112a of the capacitor 112 is connected to the ground electrode and the capacitor 112 functions as a capacitance. Similarly, when the switch 15 is turned on, the electrode 113a of the capacitor 113 is connected to the ground electrode and the capacitor 113 functions as a capacitance. Consequently, turning on the switches 14 and 15 increases the total capacitance of the LC circuit unit 104 and, from the foregoing equation 1, decreases the oscillation frequency. As above, the switches 14 and 15 can be turned on/off to change the oscillation frequency discontinuously.
As a result of this, when the capacitance switch unit 116 is operated to change the oscillation frequency stepwise and the control voltage of the variable capacitors 10 and 11 is modified to change the oscillation frequency continuously as shown in FIG. 2, it is possible to extend the range of variation in the oscillation frequency as compared to the case where the capacitance switch unit 116 is not provided, while maintaining the tuning sensitivity low to suppress variations of the oscillation frequency resulting from fluctuations in the control voltage. The provision of the capacitance switch unit 116 also makes it possible to change the oscillation frequency band arbitrarily. This facilitates providing a plurality of frequencies required for such applications as error correction in a communication system.
The foregoing conventional technology, however, has the following problem. The conventional LC-VCO 101 shown in FIG. 1 can somewhat extend the range of variation in oscillation frequency by switching the switches 14 and 15 to change the oscillation frequency in two levels as shown in FIG. 2. Turning on/off the switches 14 and 15, however, can only provide two levels of switching at best, being far from a sufficient range of variation in oscillation frequency.